Lecture 3 More OOP. CS 2113 Software Engineering Tim Wood. The George Washington University

Transcription

1 Lecture 3 More OOP CS 2113 Software Engineering Tim Wood The George Washington University

2 O.O. Design Modern software engineering is largely about designing classes, deciding how they relate, and deciding their functions + data Good design: is simple: remove unnecessary classes, functions, and data is compartmentalized: separate functionality, isolate data has clean interfaces: inputs and outputs should make sense is reusable: create general purpose code when possible!2

3 UML Unified Modeling Language Formal way to describe programs, components, functionality Designed for any object oriented programming language Defines 14 standard diagram types Structural diagrams Defines the pieces of the system and their relationships Behavioral diagrams Describe how the pieces interact!3

4 Class Diagrams Tell us how different classes are related Lists key methods and data!4

5 Simplified Class Diagrams How are these related? Human, Student, Hand, Pen, and Pencil? Human Pen Hand Student Pencil "is a" "associated with"!5

6 Class Diagram Example Class diagrams describe the software's components and how they relate Human has 2 Hand holds Pen Student owns WritingUtensil Pencil "is a" "associated with"!6

7 Arrow Direction The arrow shows which class must know something about the other A child must "know" about its parent so it can extend it The parent can be oblivious to that fact Human Hand "associated with" Student "is a (inherits from)!7

8 From Diagram to Code Human Student Hand WritingUtensil Pen Pencil public class Human { protected Hand lefthand; protected Hand righthand; public class Student extends Human { private WritingUtensil wu; public Human() { lefthand = new Hand(); righthand = new Hand(); public Student() { super(); wu = new Pen(); righthand = new Hand(wu);!8

9 Adding More Detail Full UML diagrams also include: Attributes (class data elements) Methods (class functions) Human name: String age: int lefthand: Hand righthand: Hand walk() eat() sleep() Student school: String major: String study() gotoclass() lists the data and functions added to the parent class Also uses a whole bunch of arrow types!9

10 Voice Mail System Problem description: Phone rings, is not picked up, caller invited to leave message; owner of mailbox can later retrieve message Is that enough to build the system? Requirements Engineering Science behind formally specifying what a system must do!10

11 Deciding on the parts Things: Mailboxes Messages Users Phone numbers Dates A voice mail system records calls to multiple mailboxes. The system records each message, the caller's number, and the time of the call. The owner of a mailbox can play back saved messages. Actions: Record message Replay message... Remove messages? Next/Previous Message?!11

12 MailSystem Class Diagram What are the components? What are their important functions and data? How are they related? Annotate arrows Inherits from Work with 1-2 other students and draw your diagram on the worksheet Makes use of!12

13 MailSystem Class Diagram What are the components? What are their important functions and data? How are they related? Annotate arrows Inherits from Client request: We also want to support text messages on the same system! Makes use of!13

14 How do things interact? Class diagram tells who interacts with who But doesn't illustrate how they interact UML Sequence Diagrams Show the steps required to do something!14

15 Good Practices Diagram should be loosely connected Only make a class depend on another if it really needs it Use Inheritance to replace similar functionality Red balls, green balls, and orange squares Chickens, cows, and tanks Focus on key features and level of detail UML can be a waste of time, so apply to the useful parts Iterate design and implementation Don't try to do everything the first time Build and test components in stages!15

16 UML Tool Violet UML tool makes it easy to draw diagrams Homepage: Download from 2113 Java Module 3 page! double click to edit choose mode: Select Class Inherits from... Is associated...!16

17 Banking Use VioletUML to represent the following program You have been hired by a bank to write software to track its accounts and clients. Your bank software must keep track of two kinds of accounts: Checking and Savings. Both these account types should support making deposits and withdraws. The Checking account should also allow customers to write a check and the Savings account should support adding interest. The bank needs to be able to keep track of all of its customers, each of whom may have one or more accounts. Every month, the bank needs to be able to print out a list of customers and the balance inside each of their accounts. You can assume that all customers have a unique name and that bank accounts are assigned a unique ID number. Draw a UML diagram to represent the software you would design to handle this scenario. Be sure to mark the important functions and data members for each class, and use the different arrow types to indicate which classes inherit or are associated with others.!17

18 Quiz time! Hopefully you finished Java Module 2 and read through the book! Solve the questions on the worksheet Put your answers here: You may use: Java Module 2 slides The REPL editor The GitHub Gist site Your Gist address should be something like:

19 Classes and Memory UML Class Diagrams help us visualize relations Write the code for these classes Just define the functions and data members Fill in worksheet Ignore the main function and memory layout for now You do not need to fill in the method content, just put the names of methods and variables in the right places!19

20 Classes, Objects, and Memory What will memory look like after running this code? public class Vehicle { public int speed; private int acceleration; public static int count; //... Stack Address Name Contents s v v c public class Car extends Vehicle { private String model; public void turnkey(){ Address Heap Contents public static void main() { int s = 65; Vehicle v; Vehicle v2 = new Vehicle(); v2.speed = s; Car c = new Car("Honda"); c.count = 50;!20

21 Static Object Memory Stack // A simple static object. Note: all members (data and methods) // are declared with the "static" keyword. class ObjX { static int i; static void print () { System.out.println ("i=" + i); Heap // All static members // The class that has "main" public class StaticExample { public static void main (String[] argv) { // Refer to a member of ObjX using the class name. ObjX.i = 5; Globals // Call a method using the class name and dot-operator. ObjX.print();!21

22 Dynamic Object Memory Stack // Dynamic object definition class ObjX { int i; void print () { System.out.println ("i=" + i); // NO static for either member! Heap public class DynamicExample1 { public static void main (String[] argv) { // First create an instance with space from the heap. ObjX x = new ObjX (); // Now access members via the variable and the dot-operator. x.i = 5; x.print(); Globals!22

23 Static/Dynamic (HW) Stack // Static reference to dynamic object class ObjX { int i; void print () { System.out.println ("i=" + i); // same as before Heap public class DynamicExample2 { // A simple variable declaration. static ObjX x; public static void main (String[] argv) { // First create an instance, with space from the heap. x = new ObjX (); // Now access members via the variable and the dot-operator. x.i = 5; x.print(); Globals!23

24 Multiple Objects Stack // Multiple dynamic objects class ObjX { int i; void print () { System.out.println ("i=" + i); // same as before Heap public class DynamicExample3 { public static void main (String[] argv) { // Create an instance and do stuff with it. ObjX x = new ObjX (); x.i = 5; x.print(); Globals // Create another instance assigned to the same variable. x = new ObjX (); x.i = 6; x.print();!24

25 Multiple Objects 2 (HW) Stack // Multiple dynamic objects class ObjX { int i; void print () { System.out.println ("i=" + i); // same as before Heap public class DynamicExample4 { public static void main (String[] argv) { // Create an instance and do stuff with it. ObjX x = new ObjX (); x.i = 5; x.print(); Globals // Create another instance assigned to the same variable. ObjX x2 = new ObjX (); x2.i = 6; x2.print();!25

26 Arrays of Objects (HW) Stack // Multiple dynamic objects class ObjX { // same as before public class DynamicExample5 { public static void main (String[] argv) { // Make space for 4 ObjX pointers. ObjX[] xarray = new ObjX [4]; // Make each of the 4 pointers point to ObjX instances. for (int k=0; k < 4; k++) { xarray[k] = new ObjX (); // Now assign data to some of them. xarray[0].i = 5; xarray[1].i = 6; // Print all. for (ObjX x: xarray) { x.print(); Heap Globals!26

27 Arrays of Objects v2 (HW) Stack // Multiple dynamic objects class ObjX { // same as before public class DynamicExample6 { public static void main (String[] argv) { // Make space for 4 ObjX pointers. ObjX[] xarray = new ObjX [4]; // Use fancy for loop syntax. for (ObjX x: xarray) { x = new ObjX (); // Now assign data to some of them. xarray[0].i = 5; xarray[1].i = 6; // Print all using fancy for loop syntax. for (ObjX x: xarray) { x.print(); Heap Globals!27

28 Where Does it Go? In C we had to call "free()" to make sure that the memory we used was cleaned up How come we don't need to do this in Java? MyLinkedList L = new MyLinkedList(); L.add(0); L.add(1); L.add(2); L.add(3); L.remItemAt(0); L.remItemAt(0);!28

29 Garbage Collection The Java Run Time automatically tracks what objects are actively being used in memory MyLinkedList L = new MyLinkedList(); L.add(0); L.add(1); L.add(2); L.add(3); head L.remItemAt(0); L.remItemAt(0); 3!29

30 Garbage Collection The Java Run Time automatically tracks what objects are actively being used in memory If no variable references something, then that object is "lost"---can be deleted MyLinkedList L = new MyLinkedList(); L.add(0); L.add(1); L.add(2); L.add(3); head L.remItemAt(0); L.remItemAt(0); 3!30

31 How might it do this? How to find which objects on the heap are reachable (or not)? Program knows: List of all objects that have been created List of references from the Stack and Globals area!31

32 Garbage Collection The Java Run Time automatically tracks what objects are still used in memory Stack Address Name Contents List L &50000 MyLinkedList L = new MyLinkedList(); L.add(0); L.add(1); L.add(2); L.add(3); L.remItemAt(1); L.remItemAt(0); 0 Heap Address Contents head = & value=0, next=& value=1, next=& value=2, next=& value=3, next=null head 1 3 2!32

33 Mark, Sweep Basic garbage collection algorithm Goal: find objects on the heap that are not referenced by any active object Maintain a list with a reference to all heap objects Include a "referenced bit" with each object: 1=used, 0=lost Mark Phase: Start from the root known object (e.g. top of stack) Set referenced bit to 1 for every object it references Sweep Phase: Step through list of all objects, delete anything not referenced!33

34 Garbage Collection The Java Run Time automatically tracks what objects are actively being used in memory If no variable references something, then that object is "lost"---can be deleted Stack MyLinkedList L = new MyLinkedList(); L.add(0); L.add(1); L.add(2); Heap L.add(3); Address Name Contents List L &50000 Address Reachable? Contents head = &50040 L.remItemAt(1); L.remItemAt(0); value=0, next=& value=1, next=& value=2, next=& value=3, next=null head !34

35 Benefits and Costs of GC Benefits: No "memory leaks" from forgetting to free Tighter control helps security Drawbacks: May need to completely stop application while running garbage collection Leads to unpredictable performance Newer garbage collectors support parallelism Just because there is a reference, doesn't mean it will be actively used again in the future!35

36 !36